Poly(ethylene terephthalate), sometimes abbreviated to PET throughout this disclosure, has many industrial uses, including the manufacture of fibers, films, and shaped articles. PET resins having a low to moderate molecular weight, M.sub.n =12,000-16,000, are easy to fabricate in all types of equipment. Higher molecular weight resins, for example, those having M.sub.n of about 25,000-45,000, are more difficult to extrude because more energy is required to move the viscous melt throughout the equipment.
A known expedient, that has been used in the past, is to increase the molecular weight of polyester resins directly in the fabricating equipment, for example, by coupling with an organic diisocyanate and crosslinking. Since, however, at the normal PET melt processing temperatures of about 265.degree.-270.degree. C. the intermediate --COOCONH-- group formed by reaction of the isocyanate with the terminal carboxyl groups of the poly(ethylene terephthalate) decomposes with carbon dioxide evolution, the polymer is likely to contain trapped gas bubbles. This naturally is undesirable where a homogeneous and strong material is required. A practical method of extruding various shapes from high molecular weight PET thus is needed.